Intelligent transportation systems may involve data collection, toll collection, vehicle classification, weigh in motion (WIM), and other traffic monitoring or traffic management systems.
For example, WIM systems are used to detect and weigh vehicles in motion in order to enhance the operation of road systems in a safer and more efficient manner.
A WIM system uses one or more sensors to obtain information about a vehicle as it is sensed by the sensor, typically as the vehicle moves over the sensor. Some information may be measured directly from a sensor, and other information may be measured and derived from a combination of sensors operating together.
Vehicle information that may be measured includes, for example, the number of axles, weight per axle, weight per wheel, vehicle weight, wheel count, wheel spacing, axle spacing, inter-axle spacing, axle width, and axle and/or vehicle speed. Aggregate information may also be collected such as the total number of vehicles detected by the sensors.
Time Domain Reflectometry
Generally, time domain reflectometry (TDR) is a measurement technique based on the principle that a transmission line of a particular geometry presents a known characteristic impedance. Therefore, changes to the geometry of the transmission line result in changes in the characteristic impedance that can be measured using TDR techniques. A skilled person would understand that time domain reflectometry may be used with optical or electrical signals, and that practically the optical and electrical signals are physically different requiring differently skilled knowledge and equipment to measure changes in transmission line characteristics.
In an electrical transmission line, a reflection will be generated whenever an incident wave meets a change in the characteristic impedance, which is also known as a discontinuity. TDR measurement techniques can then be used to determine the location and magnitude of the discontinuity in the transmission line from the reflected wave. Thus, the time the reflected wave takes to travel back along the transmission line can be translated into a distance along the transmission line. The magnitude of the voltage of the reflected wave can be used to calculate the amount of change in the characteristic impedance.
TDR measurement techniques may use a step input voltage for the incident wave shape as it eases the complexity of interpreting the reflected signals. In source- or both-ends terminated transmission lines, the step input voltage is divided between the source impedance and the transmission line impedance. If the source and transmission line impedances are matched, then the voltage measured between the source and the transmission line over the round-trip of the incident wave along the transmission line is half of the step input voltage. Where discontinuities exist on the transmission line, the voltage measured will deviate from exactly half due to the received reflections. Other approaches for TDR measurement may also be used, such as wave modulation with a swept frequency.
UK patent application GB 2,250,813A discloses a weighing apparatus for vehicles. The apparatus comprises a fibre optic cable whose light transmission characteristics vary under load and is encased in a pressure pad of resilient material and laid across a roadway. As a vehicle crosses the pressure pad, a time domain reflectometer calculates the load exerted by each wheel by monitoring the intensity of back scattered light from the fibre optic cable.
Known traffic monitoring and transportation management systems typically use strain gauge type sensors, for example, a mechanical strain gauge or piezo electric strain sensor, that are not configured as a transmission line. Therefore, existing intelligent transportation systems using strain gauge type sensors have signal processing systems and digital processing systems that do not utilize electrical TDR (ETDR) measurement techniques.